How events are generated in Java?

Question

First of all I am not asking about Event Handling. I know handling is implemented using the Observer pattern.

Let me take a small example. Suppose I have a Jbutton on a JFrame. I click on top of this button. Now how does the button know that I have clicked it?

1) Is there any java thread waiting for the click? If so from where does this code come from (wait till I get clicked part)? Then Does each and every swing component is waiting for events on top of threads? I assume this is a very expensive task.

2) If not, how does this work?

Answer 1

The observer pattern is used throughout the entire stack:

1. The user releases the mouse button
2. The mouse sends a message to the CPU, which triggers a hardware interrupt
3. The operating system's interrupt handler realizes that the mouse has not been moved since the button was pressed, i.e. that a mouse click occured. It identifies the window at the mouse position, and the application responsible for that window, and puts a mouse-clicked message into the application's event queue

4. Our Swing application's "event dispatch thread" runs a loop of the form:

   while (!shutdownRequested) {
       Event e = retrieveEventFromEventQueue(); // for instance our mouse clicked event
       handleEvent(e);
   }
   

   In AWT / Swing, there is single thread executing that code. The first call will block until a new event becomes available, and handleEvent() will call the listeners for this event. That is, a single thread performs all UI updates (which is why long-running tasks should not be done in event listeners, as this freezes the ui), and is sleeping if the user does not interact with the application.